ORGANIC LETTERS
Synthesis of a New Class of Carborane-Containing Star-Shaped Molecules via Silicon Tetrachloride Promoted Cyclotrimerization Reactions
2008 Vol. 10, No. 11 2247-2250
Barada Prasanna Dash,† Rashmirekha Satapathy,† John A. Maguire,‡ and Narayan S. Hosmane*,† Department of Chemistry and Biochemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115-2862, and Department of Chemistry, Southern Methodist UniVersity, Dallas, Texas 75275-0314
[email protected] Received March 24, 2008
ABSTRACT
Symmetrical star-shaped molecules with carborane clusters on the periphery have been synthesized in good yields via silicon tetrachloride mediated cyclotrimerization reactions of 9-benzyl derivatives of carboranes with acetyl group substitution on the benzene ring. Facile functionalization of these symmetrical core structures with 1-iodoheptane and trivinylchlorosilane produce compounds which could be used as liquid crystalline substances and precursors for synthesis of higher order carbosilane dendrons.
Polyhedral carborane clusters are a class of synthetically useful molecules that have practical applications in areas such †
Northern Illinois University. Southern Methodist University. (1) Zhu, Y.; Cheng, K. Y.; Maguire, J. A.; Hosmane, N. S. Curr. Chem. Biol. 2007, 1, 141–149. Hawthorne, M. F.; Mderna, A. Chem. ReV. 1999, 99, 3421–3434. Plesek, J. Chem. ReV. 1992, 92, 269–278. Teixidor, F.; Vinas, C.; Demonceau, A.; Nunez, R. Pure Appl. Chem. 2003, 75, 1305– 1313. Grimes, R. N. J. Chem. Educ. 2004, 81, 657–672. Salt, C.; Lennox, A. J.; Takagaki, M.; Maguire, J. A.; Hosmane, N. S. Russ. Chem. Bull. Int. Ed. 2004, 53, 1871–1888. ‡
10.1021/ol8005248 CCC: $40.75 Published on Web 05/09/2008
2008 American Chemical Society
as materials science, medicine, and catalysis.1 They have been widely used in making new boron neutron capture therapy (BNCT) drugs2 and, more recently, in the syntheses of macromolecular structures such as dendrimers,3 liquid crystalline substances,4 and thermally stable polymers.5 Dendrimers are globular macromolecules consisting of a symmetrical core and layers of branched repeating units extending from the core and end groups on the outer layer of the repeating units. These branched macromolecules have found applications as low-dielectric materials, catalysts,
biosensors, magnetic resonance imaging agents, drug delivery vehicles, organic semiconductors, and organic light-emitting diodes.6 Silicon tetrachloride mediated cyclotrimerization of aryl ketones has been found to be an efficient methodology for the construction of symmetrical dendritic structures.7 In this paper, we report the syntheses of carborane-containing symmetrical star-shaped molecules which can be used as a core for dendrimers and liquid crystalline substances via silicon tetrachloride mediated trimerization of acetyl-containing benzyl derivatives of carboranes. 1,3,5-Tris(1,12-dicarbacloso-dodecaboran-1-yl) benzene, a similar kind of starshaped molecule containing p-carborane, is known.8 Palladium-catalyzed cross-coupling of three 1,12-dicarba-closo-dodecaborane units to a 1,3,5-triiodobenzene afforded 1,3,5-tris(1,12-dicarba-closo-dodecaboran-1-yl) benzene in a moderate 56% yield.8 In addition, another cyclic trimer containing m-carborane is also reported to be formed in 5% yield from the reaction between 1,7Cu2C2B10H10 and meta-diiodobenzene in the presence of pyridine in 1,2-dimethoxyethane.9 Organoboranes in which the organic moiety is attached at the boron atom of the o- and m-carborane cages have been synthesized by electrophilic alkylation of carborane cages with alkyl halides and benzyl halides that contain electronwithdrawing substituents such as NO2, COOH, COOMe, and COPh in the presence of aluminum chloride.10 Employing a similar methodology, we synthesized the benzyl derivative of the o-carboranes (with benzylation at the B-9 position) containing an acetyl group on the benzene ring, as shown in Scheme 1. Electrophilic benzylation of the o-carborane and its derivatives were carried out by refluxing a mixture of (2) Yinghuai, Z.; Peng, A. T.; Carpenter, K.; Maguire, J. A.; Hosmane, N. S.; Takagaki, M. J. Am. Chem. Soc. 2005, 127, 9875–9880. Das, B. C.; Kabalka, G. W.; Srivastava, R. R.; Bao, W.; Das, S.; Li, G. J. Organomet. Chem. 2000, 614-615, 255–261. Watson-Clark, R. A.; Banquerigo, M. L.; Shelly, K.; Hawthorne, M. F.; Branh, E. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 2531–2534. Feakes, D. A.; Shelly, K.; Knobler, C. B.; Hawthorne, M. F. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 3029–3033. (3) Galie, K. M.; Mollard, A.; Zharov, I. Inorg. Chem. 2006, 45, 7815– 7820. Parrott, M. C.; Marchington, E. B.; Valliant, J. F.; Adronov, A. J. Am. Chem. Soc. 2005, 127, 12081–12089. Parrott, M. C.; Valliant, J. F.; Adronov, A. Langmuir 2006, 22, 5251–5255. Benhabbour, S. R.; Parrott, M. C.; Gratton, S. E. A.; Adronov, A. Macromolecules 2007, 40, 5678– 5688. Yao, H.; Grimes, R. N.; Corsini, M.; Zanello, P. Organometallics 2003, 22, 4381–4383. (4) Ringstrand, B.; Balinski, A.; Franken, A.; Kaszynski, P. Inorg. Chem. 2005, 44, 9561–9566. Nagamine, T.; Januszko, A.; Kaszynski, P.; Ohta, K.; Endo, Y. J. Mater. Chem. 2006, 6, 3836–3843. (5) Simon, Y. C.; Ohm, C.; Zimny, M. J.; Coughlin, E. B. Macromolecules 2007, 40, 5628–5630. Hao, E.; Fabre, B.; Fronczeka, F. R.; Vicente, M. G. H. Chem. Commun. 2007, 40, 4387–4389. Farha, O. K.; Spokoyny, A. M.; Mulfort, K. L.; Hawthorne, M. F.; Mirkin, C. A.; Hupp, J. T. J. Am. Chem. Soc. 2007, 129, 12680–12681. (6) Grayson, S. M.; Frechet, J. M. J. Chem. ReV. 2001, 101, 3819–3867. Nishimaya, N.; Iriyama, A.; Jang, W.-D.; Miyata, K.; Itaka, K.; Inoue, Y.; Takahashi, H.; Yanagi, Y.; Tamaki, Y.; Koyama, H.; Kataoka, K. Nat. Mater. 2005, 4, 934–941. (7) Bao, C.; Lu, R.; Jin, M.; Xue, P.; Tan, C.; Xu, T.; Liu, G.; Zhao, Y. Chem.—Eur. J. 2006, 12, 3287–3294. Yang, J.-X.; Tao, X.-T.; Yuan, C. X.; Yan, X. Y.; Wang, L.; Liu, Z.; Ren, Y.; Jiang, M. H. J. Am. Chem. Soc. 2005, 127, 3278–3279. (8) Schober, U.; Magnera, T. F.; Harrison, R. M.; Fleischer, F.; Pflug, J. L.; Schwab, P. F. H.; Meng, X.; Lipiak, D.; Noll, B. C.; Allured, V. S.; Rudalevige, T.; Lee, S.; Michl, J. J. Am. Chem. Soc. 1997, 119, 3907– 3917. (9) Clegg, W.; Gill, W. R.; MacBride, J. A. H.; Wade, K. Angew. Chem., Int. Ed. Engl. 1993, 32, 1328–1329. (10) Zakharkin, L. I.; Olshevskaya, V. A. Russ. Chem. Bull. 1995, 44, 1099–1101, and references therein. 2248
Scheme 1. Benzylation of o-Carborane
1-(4-(bromomethyl)phenyl)ethanone (1)11 and o-carborane in dichloromethane in the presence of aluminum chloride (Scheme 1). Although the boron atoms 8, 9, 10, and 12 of the cage are the most negative, the electrophilic reactions of 1, 2-C2B10H12 in the presence of Lewis acid catalysts such as aluminum chloride preferentially gives B-9 derivatives. The carbon atoms and those borons directly linked to them (3, 6, 4, 5, 7, 11) do not undergo Friedel-Crafts-type reactions.10,12 The cage appended 9-benzyl carboranes, 2a-b, then underwent facile trimerization with silicon tetrachloride and ethanol to produce compounds 3a-b (Scheme 2). Such symmetrical compounds are useful build-
Scheme 2. Cyclotrimerization of Benzylated Carboranes Containing an Acetyl Group on the Benzene Ring
ing blocks for designing more complex target molecules and can also serve as central cores for the design of higher level dendrimeric structures. Some of these star-shaped molecules show luminescence and liquid crystalline properties.13 FuncOrg. Lett., Vol. 10, No. 11, 2008
lithium salts of 3b with stoichiometric amounts of 1-iodoheptane in THF and the reaction of the dilithium salt of 3a with trivinylchlorosilane in THF. These reactions led to the formation of compounds 4a, 4b, and 5, respectively, in good yield. Such heptyl derivatives of carboranes have shown liquid crystalline properties,14and compound 5 may be useful for synthesis of higher order carbosilane dendrons via hydrosilylation reactions.15 All compounds were characterized by 1H, 13C, 11B NMR, and IR, and the melting points of all solid compounds were determined. Cyclotrimers 3a
and 3b especially have high melting temperatures, 180 and 170 °C, respectively. The IR spectra of compounds containing closo clusters 2-5 show strong bands between 2597 and 2569 cm-1 corresponding to ν (B-H). For compounds 2a and 2b, bands were observed at 1663 and 1666 cm-1 corresponding to ν (CdO). The 1H NMR spectra of cyclotrimers 3a and 3b and their derivatives 4a, 4b, and 5 showed resonances in the region δ 7.8-6.9 ppm typical for phenyl protons; compound 5 showed resonances between 6.28 and 5.71 ppm corresponding to vinyl protons. 13C NMR spectra of compounds 3-5 showed resonances between 145 and 123 ppm typical for phenyl carbons, and compound 5 showed resonances at 138.0 and 131.9 ppm corresponding to vinyl carbons. The proton-decoupled and proton-coupled 11B NMR spectra of all compounds exhibit a spectral pattern that corroborates substitution at the B-9 position of the carborane cages.10,12 Finally, the mass spectral data of all compounds from 3-5 have confirmed their formation. An attempt to trimerize 1-acetyl-o-carborane with silicon tetrachloride and ethanol failed to produce the desired cyclotrimer. Mechanistically, the trimerizations proceed via the initial formation of a carbocation on the acetyl group, which then reacts with another acetyl group in its enol form, and ultimately the trimer forms by combining with a third acetyl carbocation.16 From the investigations of carborane chemistry, it has been concluded that the C-H’s of the carboranes behave as electron-deficient aliphatic moieties, whereas the chemical behavior of the B-H’s has more of an aromatic character.12 The electrondeficient character of cage C-H’s may not support the formation of an adjacent carbocation. However, trimerization of 2a,b proceeds smoothly. In summary, we have shown that an acetyl aromatic moiety attached to the boron atom of the carborane cages undergoes trimerization reactions to generate symmetric building blocks for synthesis of higher order dendrimers and liquid crystalline substances, metallacarboranes, and new BNCT agents. Dendrimers are prepared using a divergent or a convergent approach. In the divergent method, dendrimers are made in a stepwise manner from a small polyfunctional core, whereas in the convergent approach, the growth of the molecule starts from the periphery and finally attaches to the core moiety.17 The present study has opened a new route to synthesize dendritic structures by both convergent and divergent approaches because of the ease of functionalization of the carboranes present on the periphery of the symmetrical core. A new class of carborane-containing carbosilane dendrons can also be synthesized starting from 5 and lithiated carboranes via hydrosilylation reactions.18 Furthermore, compounds 3a and 3b can be used to make a new class of
(11) Pieck, J. C.; Kuch, D.; Grolle, F.; Linne, U.; Haas, C.; Carel, T. J. Am. Chem. Soc. 2006, 128, 1404–1405. (12) Grimes, R. N. Carboranes; Academic Press: New York, 1970, and references therein. (13) Lo, S.-C.; Burn, P. L. Chem. ReV. 2007, 107, 1097–1116. Cao, X.-Y.; Liu, X.-H.; Zhou, X.-H.; Zhang, Y.; Jiang, Y.; Cao, Y.; Cui, Y.-X.; Pei, J. J. Org. Chem. 2004, 69, 6050–6058, and references therein. Kotha, S.; Kasinath, D.; Lahiri, K.; Sunoj, R. B. Eur. J. Org. Chem. 2004, 4003– 4013, and references therein. Kaafarani, B. R.; Wex, B.; Wang, F.; Catanescu, O.; Chien, L. C.; Neckers, D. C. J. Org. Chem. 2003, 68, 5377– 5380.
(14) Kaszynski, P.; Pakhomov, S.; Tesh, K. F.; Young, V. G., Jr. Inorg. Chem. 2001, 40, 6622–6631. (15) Gonzalez-Campo, A.; Vinas, C.; Teixidor, F.; Nunez, R.; Sillanpaa, R.; Kivekas, R. Macromolecules 2007, 40, 5644–5652. (16) Gupta, H. K.; Reginato, N.; Ogini, F. O.; Brydges, S.; McGlinchey, M. J. Can. J. Chem. 2002, 80, 1546–1554. (17) Majoral, J.-P.; Caminade, A.-N. Chem. ReV. 1999, 99, 845–880. (18) Nunez, R.; Gonzalez-Campo, A.; Anna Laromaine, A.; Teixidor, F.; Sillanpaa, R.; Kivekas, R.; Vinas, C. Org. Lett. 2006, 8, 4549–4552. Nunez, R.; Gonzalez, A.; Vinas, C.; Teixidor, F.; Sillanpaa, R.; Kivekas, R. Org. Lett. 2005, 7, 231–233. Nunez, R.; Gonzalez-Campo, A.; Vinas,
tionalization of the carborane-containing symmetrical trimers 3a and 3b may be achieved by reaction with various silanes and aliphatic halides. Scheme 3 illustrates the reaction of
Scheme 3. Functionalization of Cyclotrimers
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metallacarboranes via decapitation of carborane cages.19 A new class of BNCT agents may be possible by making these compounds water soluble.20 Further work is now underway to synthesize higher generation dendrimers containing multiple carborane clusters, derivatization of the carborane clusters on the periphery of the trimers, as well as making a new class of metallacarboranes and BNCT agents.
University) for providing the EI mass spectral data. We also thank Laura Murdaugh (Department of Chemistry and Biochemistry, Northern Illinois University) for assistance during the LCMS analysis. This work was supported by the grants from the Robert A. Welch Foundation (N-1322) and the National Science Foundation (CHE-0601023).
Acknowledgment. We thank Yuriy Pyatkivskyy (Department of Chemistry and Biochemistry, Northern Illinois
Supporting Information Available: Detailed description of typical experimental procedures and characterization data for all compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
C.; Teixidor, F.; Sillanpaa, R.; Kivekas, R. Organometallics 2005, 24, 6351– 6357. (19) Thomas, J.; Hawthrone, M. F. Chem. Commun. 2001, 1884–1885. Yoo, J.; Hwang, J.-W.; Do, Y. Inorg. Chem. 2001, 40, 568–570. Vinas, C.; Benakki, R.; Angles, P.; Meliani, H.; Teixidor, F.; Kivekas, R.; Sillanpaa, R. J. Organomet. Chem. 1998, 570, 79–87.
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